Continuous digester with dual charging means

ABSTRACT

A CONTINUOUS CELLULOSE DIGESTER EQUIPPED WITH TWO CHARGING DEVICES, ONE DELIVERING A CELLULOSIC MATERIAL INTO THE CENTER AREA OF THE INLET END OF THE DIGESTER, AND THE OTHER DISTRIBUTING A CELLULOSIC MATERIAL OVER THE SURROUNDING ANNULAR AREA. DIFFERENT DIGESTING CONDITIONS ARE EMPLOYED AND DIFFERENT CELLULOSIC MATERIALS MAY BE USED IN THE CENTRAL AREA THAN IN THE SURROUNDING ANNULAR AREA.

United States Patent Ofhce U.S. Cl. 162-237 3 Claims ABSTRACT OF THE DISCLOSURE A continuous cellulose digester equipped with two charging devices, one delivering a cellulosic material into the center area of the inlet end of the digester, and the other distributing a cellulosic material over the surrounding annular area. Different digesting conditions are employed and different cellulosic materials may be used in the central area than in the surrounding annular area.

The invention relates to a continuous cellulose digester of the kind having means for causing a plug of a finely comminuted fiber material to move with a velocity uniformly distributed over the cross-section of the digester, through a digesting zone adapted for fiber-liberating digestion of the fiber material. In upright cylindrical digestcrs having charging and discharging devices for the fiber material at their opposite ends, the fiber material will move as a solid plug, i.e. Without whirling or tumbling movements therein, even when digesting and washing liquids are driven through the fiber material column transversely thereof and/or in co-current or counter-current relatively thereto. Usually a product digested as evenly as possible is desired, but according to the present invention the abovementioned rigidity of the fiber material column is employed so as to have the fiber-liberating digestion in one and the same digester take place differently in various parts of the digester cross-section.

The essential characterizing feature of the invention contemplates designing the digester to maintain substantially different digesting conditions in the center of the cylindrical plug than in the outer portions thereof. This means that the fiber material passing close to the axis of the digester (the core of the plug) yields pulp of a different quality than the fiber material passing close to the shell of the digester (the sheath of the plug). The principle of invention may be used for obtaining a final product of the digestion consisting of two kinds of pulps suitable for different use, Ibut another possible use may be to digest two dissimilar starting materials differently in order to have the qualities of the pulp kinds optained approach each other, so that a mixture thereof can take place in connection with the discharge thereof from the digester.

According to the invention the digester is preferably designed in such a manner as to bring about a pronounced discontinuity in respect of a factor influencing the digesting reaction in a layer of the fiber material plug distant from the digester shell and having the approximate shape of a cylindrical face coaxial thereto, whereby on opposite sides of said border layer, pulps with clearly different qualities are obtained. Too wide a zone of transition should be avoided. Above all said discontinuity may concern one or more of the following factors: the ternperature, the strength or concentration and the liow rate of the digesting liquor used, but may also concern the quality of the fiber material, such as different wood species of different particle sizes.

3,554,864 Patented Jan. 12, 1971 When two different fiber materials, such as chips of coniferous wood species and chips of deciduous wood species, and digested in the one and same digester, then according to the invention the digester is equipped with devices for bringing about a concentric distribution of the fiber materials, so that a core of the fiber material column is formed out of the first material, said core being surrounded on all sides by the second fiber material.

If insteadthereof or simultaneously therewith the discontinuity should be caused by a modification of the `known liquor circulating system, there is provided a sieve for withdrawal of circulating digesting liquor at a considerably smaller radius than that of the digester shell. As an alternative thereto, the orifices through which digesting or washing liquids are spread out in the digester may be placed off center in order to leave a central core of the chips column more or less uninfiuenced.

The invention will be more closely described herein below with reference to accompanying drawings, in which FIG. 1 diagrammatically shows a continuous cellulose digester to Iwhich the invention is applied, and fFlG. 2 shows a modification thereof.

`On the whole the digester shown is of the design described in the U.S. Pat. No. 3,298,899. It is adapted for the performance of a fiber liberating digestion of lignocellulosic fiber material followed by liquor removal and a first washing step of the fiber material taking place already in the digester. Thus the digester consists of an upright, at least substantially cylindrical (or somewhat conical) container 11, in which the fiber material moves continuously in the downward direction as a coherent column or plug while various treatments are performed thereupon. The treated material which may be wood chips, shredded straw or other finely comminuted vegetabilic fiber material, is supplied to the top of the digester at such a rate that the digester is held almost filled therewith. To the upper end of the digester high-pressure steam is supplied through the conduit 17 in order to heat the fiber material and possibly purge the same of air. Gas collected in the digester top is let out through a conduit 19 having a control valve 21 by means of which the pressure of the digester is controlled so to maintain a value of approximately 8 kgs/cm?.

Digesting liquor, such as sulphate liquor, which is supplied by a conduit 23, is pumped 'by means of a pump 25 through a heat exchanger 27 wherein the liquor is heated to digesting temperature, and then into the digester through a conduit 29 opening out centrally therein. The liquor is spread radially in all directions and pushes before itself such earlier supplied liquor as has already been somewhat cooled by the fibre material, towards a sieve girdle 31 inserted in the digester shell. Said sieve girdle is connected to the inlet side of the pump 25, the displaced liquor being recirculated after heating.

The charging speed of the fiber material is so chosen that the digesting reaction is finished when the material reaches the level of the sieve girdle 35 inserted in the digester shell. Spent liquor separated from the pulp by said sieve girdle is discharged via a control valve 37 into a flash tank 39, wherefrom the liquor preferably is sent to a plant for recovery of the chemicals thereof. By means of the pump 41 and a tube 43 opening out centrally in the digester the spent liquor is held in circulation in order to equalize the concentration at the level of the sieve girdle 35.

In the lower part of the digester a first washing operation of the digested pulp is performed by means of washing liquid, e.g. water or filtrate, taken from the conduit 45 and pumped by means of the pump 47 through the tube 48 opening out centrally in the digester. A sieve `girdle 49 inserted in the digester shell near the lower end thereof is connected to the inlet side of the pump 47, so that a circulation is maintained whereby the wash liquid is spread radially and distributed over the entire digester cross-section at the level of the sieve girdle 49. From there the wash liquid moves counter-currently to the ber material towards the sieve girdle 35 while replacing and displacing the digesting liquor. Part of the wash liquid accompanies the spent liquor out through the sieve girdle 35. The wash liquid supplied also serves to cool the pulp in the bottom end of the digester.

According to the invention, the abovedescribed digester is equipped with two separate systems for the supply of fiber material. One of them comprises the supply conduit 12 and the charging valve 14 which may consist of a rotary pocketed wheel, and the other system comprises the supply conduit 16 and a similar charging valve 18. The valve 14 is located eccentrically upon the conical top of the digester and is connected to a short tube 20 located inside the digester, which tube is inclined downwardly and inwardly and delivers the material admitted by the valve 14, within the central part of the digester cross-section. The second charging valve 18 is located centrally upon the digester top, and in the digester below said valve there is placed a conical guide plate 22 which is concentric to the digester top and which diverts the material admitted by the valve 18 sidewise with an even distribution in all directions. When ber material is admitted by the two valves 14, 18 at a correct and mutually adapted rate, the material supplied through the valve 14 will drop down into and till out a central zone of the digester cross-section, whereas the material supplied through the valve 18 is spread over a peripheral zone thereof. In the fiber material column thus built up the particles maintain their relative positions during the continuous movement thereof towards the digester bottom. Thus, there is formed a cylindrical border face or layer 33 between the central zone A and the peripheral zone B, i.e. between material from one and material from the other changing system. The diameter of said border face depends upon the relative charging rates. Of course, it is hardly possible to maintain any quite sharp border, but there is formed a blending zone the size of which, however, can be limited by a proper choice of the size and location of the guide plate 22 and by correctly controlling the level of the level of the filling of the digester with solid material.

At its lower end the digester is provided with two separate outlets each having its own discharging device. The two outlets are provided upon a particular bottom piece 51 fastened by means of a flange joint 53 to a neck S5 forming the lower end of the digester. One outlet is formed by a circular groove 57 in the upper face of said bottom piece, and a passage 59 extending from said groove rst downwardly and then radially outwards. Connected to said passage 59 is a first pulp conduit 61 wherein a throttle valve 63 is inserted for the control of the discharging speed. The second outlet is formed by a similar circular groove 65 shaped in the upper face of the bottom piece and concentric to the groove 57 but having a greater diameter than the latter, and a connected passage 67 extending from the bottom of the groove first downwardly and then radially outwards in a direction opposite to the direction of the passage 59 and connected to a second pulp conduit 69 having a throttle valve 71.

The bottom piece 51 has a vertical bore coaxial to the digester, and through said bore there pass a sleeve-shaped driving shaft 73 and a solid shaft 75 inserted in the former and coaxial thereto. The shafts 73, 75 are sealed to the bottom piece and to cach other by means of stung boxes 77, 79 and are driven each by a separate motor 81, 83 via a gear. Attached to the upper end of the shaft 75 is a scraper 85 having a hub 87 and essentially radially therefrom extending arm 89 which carry obliquely set scraper blades 91. The diameter of the scraper 85 is considerably less than the diameter of the digester shell and corresponds lo the diameter of the core A of the chips column which should be discharged separately. The sleeve-like shaft 73 carries a second scraper 93 having a greater diameter in order to be able to act upon the pulp in the peripheral zone B. From the hub of said scraper there extend radial arms 97 which extend beyond the periphery of the scraper and reach forth to the vincity of the cylindrical shell of the digester. Said arms carry obliquely set scraper blades 95, 99 which move along the digester bottom and forward pulp down into the groove 65, from where the pulp is scraped down into the outlet passage 67 by means of a scraper blade 101. The hub of the lower scraper also carries a bafe or screen sleeve 103 concentric to the digester shell and serving to prevent cornmunication between the core zone and the peripheral zone in the digester bottom end and particularly in its tapering neck portion. Said screen sleeve comprises a lower cylindrical portion which moves close to an annular ange 105 shaped on the bottom piece 51 between the grooves 57 and 65. At its upper end the screen sleeve widens lik'e a funnel to a maximum diameter approximately equal to the diameter of the upper scraper 85, where the screen sleeve is integrally united with the scraper arms 97 which admit passage of the pulp between themselves. Between the screen sleeve and the hub there are passages 107 through which the pulp caught within the screen sleeve during the continuous descent of the pulp column, passes down into the groove 57 in the bottom piece. The scraper blades 91 of the upper scraper move quite close to the upper side of the screen sleeve and assist in the downward feed of the pulp through said passages 107. From the groove 57 the pulp is moved into the outlet passage 59 by means of a scraper blade 109 attached to the lower scraper.

With the device above described it is possible to discharge through the outlet 59 such pulp as has been obtained after digestion exclusively or mainly of such starting material as is supplied through the conduit 16, and simultaneously to discharge through the outlet 67 pulp obtained exclusively or mainly after digestion of such starting material as is supplied through the conduit 13. The blend pulp obtained in the layer 33 can be discharged at will through one or the other outlet by choosing a corresponding size of the working zone of the central scraper.

The two kinds of ber materials are digested in the same digesting liquor and essentially at the same temperature. By properly adapting the heating taking place in the heat exchanger 27 relatively to the quantity of circulating liquor it is possible, however, to maintain a higher temperature in the central zone A than in the peripheral zone B, e.g. approximately 170 C. in the former and approximately 160 C. in the latter. However, there will be no sharp temperature gradient.

The modification shown in FIG. 2 differs from the embodiment above described merely in that the sieve girdle 31 has been replaced by a cylindrical sieve 115 which forms the inner wall of the double-walled annular hollow body 117 having a smaller diameter than the diameter of the shell 11 of the digester and being concentric to the digester. The digesting liquor which penetrates through the sieve and enters the interior of the hollow body is led by a conduit 119 inserted radially in the digester and extending out through its shell, to the pump 25 and then on in circulation. Thus the liquor circulation does not extend to the peripheral parts of the digester cross-section, which are situated radially outside the sieve 115, and therefore the digestion will take place in these peripheral parts at a considerably lower temperature than in the core zone.

Thus in this case, the two kinds of fiber materials will be digested differently and the respective digesting temperatures can be adapted to the quality of the fiber materials in order to yield the desired pulp qualities.

For instance, the two kinds of fiber materials supplied to the digester may consist of chips of different wood species or of chips of the same wood species but of different size classes. One kind or both kinds may be pretreated in one way or other, for instance steamed and/ or impregnated with liquors of the same or different kinds or concentrations. The supply of liquor through the conduit 23 may then be superfluous or may serve merely to control the liquor strength in the central digesting zone A.

With the arrangement shown in FIG. 2 it is also possible to obtain from one and the same starting material two kinds of pulps that are somewhat dissimilar. In this embodiment of the fiber material is supplied through both of the conduits 12 and 16, and the quality difference between the pulp kinds digested centrally and peripherally depends merely upon the different digesting temperature. As an alternative, the embodiment shown in FIG. 2 may be modified and simplified by replacing the two charging systems by a single one. t

Also in other respects the above described embodiments may be modified as to their details, within the scope of the following claims.

What is claimed is:

,1. An upright cylindrical cellulose digester having a digesting zone including a central portion extending axially of the digester and a peripheral portion surrounding said central portion, supply means for feeding finely comminuted fiber material into the top of said digester and means for moving said material in the form of a plug through said digesting zone at a velocity uniformly dis-v tributed over the cross-section of the digester to effect fiber-liberating digestion of said fiber material, said supply means including dual charging means for feeding two separate supplies of fiber material into said digester, the first of said charging means communicating with the central portion of the digester to deliver a first supply of fiber material thereto and said second charging means communicating with the peripheral portion of said digester to deliver a second supply of fiber material thereto whereby the first supply of material forms a core of the liber material plug moving axially through the digester and the second supply forms a peripheral sheath enclosing said core on all sides.

2. A digester according to claim 1 including deilecting means positioned in said digester near the top thereof and over the central portion of said digesting zone, the first charging means opening beneath said deflecting means to supply fiber material into said central portion of the `digester and the second charging means opening above said deflecting means to supply fiber material into the digester and onto the deecting means whereby said dcflecting means directs said material to the peripheral portion of the digester.

3. A digester according to claim 1 including a conical guiding plate positioned within said -digester adjacent the top thereof and concentric therewith, said first charging means opening into said digester beneath said plate and the second charging means opening into said digester directly above said plate.

References Cited UNITED STATES PATENTS 3,326,235 6/1967 Jones 162--246 3,372,087 3/ 1968 Richter 162-251 3,445,328 5/1969 Laakso 162-19 S. LEON BASHORE, Primary Examiner T. G. SCAVONE, Assistant Examiner U.S. Cl. X.R. 

